How to Paint a Concrete Block Retaining Wall That Weeps Moisture

Painting a concrete block retaining wall that weeps moisture is one of the trickier DIY painting jobs — but it's absolutely doable if you tackle the root cause first. Slapping paint over a damp wall without addressing drainage is the number one reason coatings fail, blister, and peel within months. Follow the steps below and you'll end up with a finish that actually lasts.

Understand Why the Wall Is Weeping

Concrete block (also called concrete masonry unit or CMU) retaining walls are porous by nature. In New Zealand's variable climate — particularly in wetter regions like the West Coast, Northland, or Wellington — groundwater and rainwater build up behind the wall and push through the blocks. This is called hydrostatic pressure, and it's the enemy of any surface coating.

Before you open a paint tin, ask yourself:

• Is there a weep hole or drainage system behind the wall? If not, water has nowhere to go but through the face.
• Is the weeping seasonal (after heavy rain) or constant? Constant weeping suggests a drainage problem that paint alone cannot fix.
• Are there cracks or crumbling mortar joints letting water in?

If the wall has no drainage and weeps constantly, you may need a builder or civil contractor to install agricultural drain pipe (ag-pipe) and gravel backfill before painting. Check with your local council whether any remedial work to a retaining wall requires a building consent — walls over a certain height often require consent under the NZ Building Code — thresholds and conditions vary, so check with your local council. The MBIE Building Performance website has guidance on retaining wall consent requirements.

Gather Your Tools and Materials

• Stiff wire brush and angle grinder with masonry cup wheel
• Pressure washer (or garden hose with high-pressure nozzle)
• Masonry crack filler or hydraulic cement
• Alkali-resistant masonry primer (critical for concrete)
• Waterproofing masonry paint or elastomeric coating
• Thick-nap roller (18–20 mm nap) and masonry brush
• Safety glasses, gloves, and P2 dust mask

Step-by-Step: Prep the Surface

1. Clear and Clean

Remove any loose paint, efflorescence (the white salt deposits), moss, and dirt. A pressure washer works well here. For stubborn efflorescence, scrub with a diluted masonry cleaner — follow the product directions carefully.

2. Repair Cracks and Joints

Use a hydraulic cement product (available at Bunnings or Mitre 10) for active leaks — these set even when water is present. Rake out crumbling mortar joints and repoint with a suitable mortar mix. Allow all repairs to cure fully, typically at least 48–72 hours.

3. Let It Dry — As Much as Possible

This is the hard part. Ideally, you want the surface as dry as possible before priming. Choose a dry spell of at least 3–5 days. If the wall is still slightly damp, use a primer specifically rated for damp or green concrete.

Choosing the Right Paint System

Using a standard exterior paint on a weeping masonry wall will fail. You need a system designed for the job:

Resene offers their Resene Concrete Primer and Resene Lumbersider or Resene Sandtex range for masonry. Dulux has their Dulux Weathershield Masonry system. Wattyl also produces exterior masonry coatings suited to NZ conditions. Always check the technical data sheet for each product — some are not suitable for below-grade or below-ground applications.

For walls with ongoing moisture pressure, consider a crystalline waterproofing product (such as those from Sika or Ardex) applied before painting. These penetrate the concrete and block water at a chemical level rather than just coating the surface.

Applying the Paint

1. Prime first — apply the alkali-resistant primer generously, working it into the block texture with a thick-nap roller and cutting in with a brush. Allow to dry per the manufacturer's instructions.
2. Apply topcoat in two coats — don't try to get full coverage in one pass. Allow the first coat to dry fully before applying the second.
3. Work in suitable conditions — avoid painting in direct hot sun, high wind, or if rain is forecast within 24 hours. NZ's UV is intense, so a UV-stable topcoat matters.
4. Don't paint the weep holes — if your wall has weep holes at the base, keep them clear so water can still escape.

Ongoing Maintenance

Even the best paint system on a retaining wall will need attention over time. Inspect annually for blistering, cracking, or new efflorescence. Touch up early rather than letting moisture get under the coating and cause widespread failure.

When to Call a Professional

If the wall weeps constantly regardless of weather, has significant structural cracking, or is of significant height, it's worth getting a qualified builder or civil contractor to assess the drainage and structural integrity before you spend money on paint. A Licensed Building Practitioner (LBP) can advise on whether any work requires consent. For the painting itself, a professional painter experienced in masonry and waterproofing coatings will get the best result on a tricky substrate. You can find qualified local painters and builders on tradedirectory.co.nz.

Safety note: When grinding or wire-brushing concrete, always wear a P2 dust mask, safety glasses, and gloves. Concrete dust can cause serious lung damage with repeated exposure — see WorkSafe NZ for guidance on working safely with masonry.

Sources & Further Reading

• MBIE Building Performance — Retaining Walls
• BRANZ — Moisture in Buildings
• Resene — Masonry Painting Guides
• Sika NZ — Waterproofing Products
• WorkSafe NZ — Hazardous Substances & Dust
• Licensed Building Practitioners